AbstractThe presence of maxima and sometimes minima in theadsorption isothenns of surfactant on minerals has beenattributed to various mechanisms involving micell~ ex-clusion, impurities, surfactant composition, mineralmorphology, etc. This study on the precipitation ofsulfonates with various inorganic ions (Na, K, NH4, Ca,Mg, and AI) and the subsequent l'eQissolution when thesurfactant concentration is increased shows a precipita-tion maximum to occur in the same surfactant concentra-tion range where an adsorption maximum is obtained.Simultaneous abstraction, precipitation, and adsorptionexperiments have been conducted using chromatograph-ically purified dodecylbenzenesulfonate and Na-kaolinite, and the results show the "abstraction vs. sur-factant concentration" curves to exhibit a much morepronounced maximum than the real adsorption (abstrac-tion minus precipitation) maximum. We suggest that thetotal abstraction of surfactant from the solution on con-tact with mineral phase, usually called "adsorption," isa summation of both the .'real' , adsorption and

precipitation and that the precipitation/redissolutionphenomenon is one of the major reasons for the adsorp-tion maxima. Introduction of oil into the system reducesthe precipitation because of panitioning of the sulfonatebetween the oil and aqueous phases. Precipitation andadsorption data are used to derive possible mechanismsfor the precipitation/redissolution phenomena.

alkylbenzenesulfonate investigated in this study showeda complex and interesting behavior involving precipita-tion followed by redissolution at higher sulfonate con-centrations. The phenomena of precipitation andredissolution were found to have a definite influence onthe nature of the adsorption or depletion. ~ote that thelatter includes surfactant lost not only by adsorption, butalso by precipitation and entrapment.

Adsorption isotherms of surfactants from micellarsolutions often exhibit a maximum and sometimes evenminimum. 1-5 The presence of maximum has been at-tributed in the past to mechanisms involving micellar ex-clusion from the interfacial region because of elec-trostatic repulsion or structural incompatibility, thepreSence of impurities, surfactant composition, adsor-bent morphology, etc.I-3 None of these mechanisms,however, is substantiated well enough to be considered aconfirmed mechanism for surfactant adsorption fromconcentrated solutions, particularly since there areserious PQssibilities of experimental artifacts arisingfrom such processes as precipitation and entrapment. In-dee<!, more than one mechanism can operate in differentsystems or in the same system under even slightly dif-ferent conditions. In .our studies, the phenomenon ofprecipitation was responsible to some extent for theamount of adsorption as well as its maximum.

The investigation of the precipitation/redissolutionprocess (in the absence of mineral) can help in develop-ing an understanding of the mechanisms involved.Precipitation experiments were therefore conducted inthis study with sulfonate solutions containing differenttypes of ions, namely, sodium, calcium, and aluminum.Kinetics of precipitation was investigated under selectedconditions by studying the deyelopment of .the solutions'turbidity. .

From the data, solubility products of the Ca and AIsulfonates are determined. Mechanis'ms for theredissolution are suggested.

IntroductionThe effect of inorganic electrolytes on the behavior ofsurfactants in porous media is of considerable interestsince reservoir fluids contain significant amounts ofvarious mono- and multivalent inorganic ions. Interac-tion of these ions with the surfactant in the bulk solutionduring micellar flooding can lead to surfactant precipita-tion and to surfactant depletion and possibly even toreservoir plugging.

Precipitation is often observed during adsorption ex-periments when surfactants are contacted with mineralsuspensions due to the presence of inorganic ions that areadded to the system or introduced by mineral dissolu-tion. The system made up of kaolinite and

Experimental Procedure

Materials. Samples of sodium dodecylbenzenesulfonatespecified to be 90% pure (purchased from LachatChemicals) were purified be deoiling, "recrystallization,and liquid chromatographic (LC) techniques. Sodium(l,butyloctyl)benzenesulfonate (Texas ~o" 2) purcbased

0117.~2S~I 1184 Society 01 P8traum EngI.-r& 01 AlME

APRlLI984 233

P. Somasundaran, SPE. Columbia U.M. Celik, SPE. Columbia U.A. Goyal, Columbia U.E. Manev, Columbia U.

SALT CONCENTRATION. MIl

1.00.1100

1.44 mOl/m3 DEOILED Ho0D8S (H 002)

KCI

T-24.CpH - 5.810.2

80

z0u; 60II)iII)Z4Eto. 40~

20

00;1 1.0

SAL T CONCENTRATION. kmol/m'

Fig. 2-Light transmission of dodecyi)enzenesulfonate solu-tions as a function of added salt concentration.

residual concentration was detenl1ined after centrifuga-tion for 20 minutes. Precipitation was calculated fromthe differences between the initial and fmal values ofsulfonate concentration. 1n certain cases, the turbidity ofthe sulfonate/supernatant mixture also was measured.

A Brinkman PC-(xx) probe colorimeter was used tomeasure turbidity of the solutions. Equal amounts (5 mL[5 cm 3]) of surfactant and inorganic electrolyte solutionswere mixed in a test tube and shaken for 10 seconds. Theprobe was then inserted directly into the 10-mL.[1O-cm3] sample and the fust reading was taken ISseconds after mixing. The dependence of turbidity onsurfactant and/or electrolyte concentration was deter-mined from the values of light transmission or opticaldensity measured 30 minutes after mixing. All turbiditymeasurements were made at room temperature (25°C[77°F]).

from the U. of Texas was used as received. Surface ten-sion vs. concentration curves obtained for aqueous solu-hons of recrystallized and LC-purified dodecylbenzene-sulfonate as well as Texas No.2 (l,butyloctyl)benzene-sulfonate obtained with the Wilhelmy plate technique didnot exhibit any minimum. The critical micelle concentra-hons (CMC) determined from the surface tensionmeasurements in the absence of addihonal electrolytewere approximately 10-3 kmol/m3 [O.84xIO-.5 Ibmmol/gal] in all cases.

The mineral used in the adsorption experiments wasNa-kaolinite, prepared from a well-crystallized samplepurchased from the clay depository at the U. of Missouriby the procedure described in Ref. 3.

Inorganic salts used in the precipitation experimentsand to adjust the ionic strength and pH were of analyticalreagent grade. HydrocartJons specified to be 99+ % purewere purchased from Aldrich Chemical Co. Inc. Triplydistilled water was used in all experiments.

Results and DiscussionKinetics of Precipitation. The rate of formation ofprecipitates depends on experimental conditions such asthe concentrations of added surfactant and inorganicelectrolytes, the type of ions p~nt, and the degree ofstirring. Fig. I illustrates the development of turbidity asmeasured by the optical density of the solutions as afunction of time when 10-~ kmol/m3 [0.84 X 10-5 Ibmmol/gal] (l,butyloctyl)benzenesulfonate solution ismixed with solutions containing various amounts ofCaCI2. We observed that at higher concentrations ofelectrolytes, development of turbidity is rapid andequilibrium is approached in minutes. At lower concen-trations, however, the initial rate of the turbiditydevelopment is relatively low and an equilibrium isreached only in hours. At very low concentmtions ofCa++ «10-4 kmol/m3 [<0.84XIO-6 Ibmmol/gal]), p~ipitate is found to appear only after

SOC1ETY OF PETROLEUM ENGINEERS JOURNAL

Test Procedures. In the adsorption tests, desiredamounts of solids fi~t were agitated on a wrist-actionshaker for 2 hou~ in the electrolyte solution. Sulfonatethen was introduced into the suspension and furtheragitated for 72 hou~. The suspension was centrifuged ata gravity force of 1,500 for 20 minutes, the liquid abovethe mineral layer was mixed by inverting the tubecarefully, and the supernatant was analyzed for theresidual concentration of the sulfonate by the two-phasetitration technique using the dimidium bromide/disul-phine blue mixed indicator as described by Powe~ 6 andReid et al. 7.8 Adsorption tests were carried out at 3O°C

[86°F).Precipitation of the sulfonate during adsorption ex-

periments by the ions present in the clay supernatant wasstudied in the following manner. A sample of the super-natant was prepared under conditions similar to thoseused in the adsorption tests described. Na-kaolinite wasequilibrated with the desired electrolyte solution at asolid-to-liquid ratio of 0.2: 1 for 72 hou~ and then thekaolinite was removed by centrifugation for 20 minutes.To avoid any dilution effects, the supernatant wasevaporated to dryness and the desired volume of surfac-tant solution was added. The test solution then ~asagitated on a wrist-action shaker for 12 hou~ and its

234

0 10"1~zNOC' CONC[NT~AT'ON. '.01/.1

Fig. 6-Light transmission as a function of added concentra-tion of NaCI for two concentrations of Na~benzenesulfonate in the presence of CaCI2 .

was added at constant surfactant concentration also canresult from the differences in kinetics of precipitationunder various conditions.

The solubility product, K s' of the sulfonate salts wascalculated with the help of the turbidity vs. log concen-tration data and using the following expression relating itto activities of ions.

Ks=[aMm+ ][as-]m

where aM m" and as - represent the activities of cationand sulfonated species, respectively. Using the data forthe concentmtions that correspond to the initial increasein turbidity and the respective activity coefficientscalculated using the Debye-Hiickel equation, thesolubility product was calculated to be 2 x 10 -I J forcalcium and 4 x 10 -19 for aluminum sulfonates.

[0.84 x 10 -5 Ibm mol/gal) sulfonate, it is apparently

unable to produce enough micelles to cause redissolutionof the precipitate panicles. Indeed, the addition of NaCIcan be expected to lower the CMC of sulfonate and thusenhance the formation of micelles and cause redissolu-tion of the precipitate. Upon funher addition of NaCI,the solubility limit of the sulfonate is exceeded and itprecipitates out of the solution. For the present cases, thesulfonate tolerance limit corres~nds to approximately0.3 kmol/m3 NaCl [O.25xIO- Ibm mol/gal).

The implication of the phenomena of precipitation andredissolution to adsorption is important since adsorptionis usually conducted in brine solutions. Release ofmultivalent ions such as Ca + + , Mg + + , or Al + + + by

the clay in the presence of NaCI can produce precipita-tion and redissolution regions which can in turn generateapparent maxima or minima in adsorption isotherms.Furthermore, precipitation of sulfonates can conceivablylead to plugging of pores in reservoirs with serious con-sequences. The potential role of redissolution in suchcases deserves investigation.

Precipitation/Redissolution in Mono- and Multi-valent Salt Mixtures. The effect of NaCI addition oncalcium sulfonate precipitate that has already been fonn-ed in solution is shown in Fig. 6 where transmission ofthe suspension is plotted as a function of the concentra-tion of NaCI. In the absence of NaCl. addition of5XIO-3 krnol/m3 [O.42XIO-3 Ibm mol/gal] CaCl~results in almost complete precipitation of 10-krnol/m3 [O.84XIO-s Ibm mol/gal] and 5xlO-3krnol/m 3 [0.42 x 10 - 3 Ibm mol/gal] sodium dodecyl-

benzenesulfonate solutions. As NaCI was added to thesesystems. tumidity gradually decreased. and finally. atabout 5XIO-2 krnol/m3 [0.42xlO-2 Ibm mol/gal] ofNaCl. a clear solution was obtained.

An explanation for this interesting effect of NaClbecomes evident when the possible role of micelliza-tion/solubilization phenomena is examined. For com-plete redissolution of calcium sulfonate precipitates. asstated earlier, sulfonate is required to be at a concentra-tion that is at least five times that of calcium (5 x 10-3krnol/m3 [0.42 x 10-3 Ibm mol/gal)) CaCI2. Since thesolution considered here contains only 10 - 3 krnol/m 3

Precipitation, Abstraction, and Adsorption.Precipitation and redissolution of the previously discuss-ed type can give rise to a maximum in the abstraction(abstraction of surfactant from solution) isothenns and,if precipitation is not totally isolated from adsorption,can generate an apparent maximum in the adsorptionisothenn. In the case of clays, a number of mono- andmultivalent cations-Na +, Ca + +, Al + + +, etc.~

be expected to be present in a supernatant of it and toproduce precipitation.

Results obtained for abstraction of as-received andpurified sodium dodecylbenzenesulfonate from a solu-tion upon the equilibration of it with homoionic Na-kaolinite are given in Figs. 7 and 8. These isothenns arecharacterized by the presence of a maximum. The datafor sulfonate depletion on contact with the supernatant ofa kaolinite suspension as well as the data for the !ight

SOCIETY OF PETROLEUM ENGINEERS JOURNAL236

RESIDUAL CONCENTRATION. mM/1

20 ~ 1~ 2~ 3,0 4,° S,O ',O '0

NoDDBS/No - KAOLINITE (A2)

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SOLID/LIQUID' 0.2T . 31 t 2.C

. CONTACT TIME' 7211 .

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RESIDUAL CONCENTRATION, mM/1

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RESIDUAL CONCENTRATION. mol/m3

Flg.7-Abstraction, precipitation, and adsorption of LC-purified Na-dodecylbenzenesulfonate on Na-kaoliniteas a function of concentration.

~ ~H. 6.0 ~ 0.30 . r"'.;.:::=::~_~~ '1100

0 10 20 30 40 50 60

RESIDUAL CONCENTRATION, mol/m3

Fig.8-Abstractk>n, adSOl'Ption, and precipitation isothennsof as-received dodecyIbenzenesulfonate on Na-kaolinite. Adsorption isotherm is compared with thatof LC-purified dodecylbenzenesulfonate.

transmission of the sulfonate supernatant mixtures beforecentrifugation also are given in these figures. Variousabstraction curves show excellent comlation; both theprecipitation and transmission curves exhibit maxima inthe concentration region of the abstraction maximum.

The amount of sulfonate that will precipitate becauseof interaction with dissolved mineral species in thepresence of kaolinite need not be precisely equal to thatwhich will precipitate in a supernatant from whichkaolinite has been removed because of elimination of thepossibility for continuous equilibration of kaolinite withthe solution in the latter case. The natural pH values ob-tained during precipitation tests were slightly higher thanthose obtained in the abstraction tests, again from lack ofcontinuous equilibration in the presence of kaolinite.Assuming these two effects to be minimal, adsorptionisotherm was calculated from the difference between thetwo isotherms (Figs. 7 and 8).

The adsorption maximum is considerably less sharp,suggesting that the precipitation of sulfonate from in-teractions with the dissolved mineral species does indeedcontribute toward determining the shape of the abstrac-tion isotherm for the system considered here. The resul-tant adsorption isotherm still exhibits a maximum, eventhough much less prominent. The presence of such max-imum could be attributed to other mechanisms whichhave been suggested earlier,I-3.13.14 but it is possiblethat this is caused by the absence of kaolinite during theprecipitation process in supernatants. As sulfonateprecipitates, when kaolinite is present in the system, and

APRIL 1984

consumes the dissolved mineral species, more ions canbe expected to be released from the mineral because ofthe offset of equilibrium conditions. In the abstractionexperiments, precipitation can therefore be higher thanwhat was observed during precipitation experiments.Release of ions can conceivably be higher when the in-itial precipitation is rapid rather than when it is slow.This can cause the decrease in the abstraction density tobe higher in the rapid precipitation region. The observeddecrease in precipitation at higher surfactant concentra-tions can be caused by the solubilization of theprecipitate by the micelles which are known to bring intosolution otherwise insoluble nonionic organic species. 12

Release of mineral species is confirmed by the surfacetension measurements made with NaCl solutions andwith clay supernatant solutions. With the dissolution ofthe mineral, ionic strength would increase, and the CMCof the sulfonate solution should decrease. The oppositewas observed here (Fig. 9); CMC in the supernatantsolution is sl,ghtly higher than in the NaCl solution. Thisis because precipitation of sulfonate had occurred in thesupernatant solution. Since precipitation is a competingphenomenon, higher total surfactant concentration willbe required for the attainment of CMC.

The effect of addition of oil to precipitating systems isillustrated in Fig. 10. Addition of n-dodecane (10 and52% by weight with respect to the sulfonate) eliminatedprecipitation in the low-surfactant-concentration region.Note that precipitation caused by salting out still occur-red in concentrated sulfonate solutions.

237

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Fig. 8-Surface tension of Lc-purified Na-dodecylbenzene-sulfonate in 0.1 kmoUm3 (0.84 x 10-3 Ibm moVgal]NaCI and in clay supernatant prepared in 0.1kmoVm3(0.84x10-3 Ibm moVgaij NaCI.

O.""'.."""""i""""';"""'~ I 'u0 10 20 30 40 50 -60

RESIDUAL CONCENTRATION. mol/m5

Fig. 1G-Effect of addition of n-dodecane on the adsofption ofNa-dodecylbenzenesulfonate on Na-kaolinite.

The elimination of precipitation possibly is caused bypartitioning of the surfactant species into the separate oilphase present in the system or the dissolution of theprecipitate in the oil that might be present in the form ofmicroemulsion. In the former case, surfactant speciescan be partitioned between the separate oil and aqueousphases, thus reducing the surfactant concentration in theaqueous phase and consequently reducing the precipita-tion also. The latter case is similar to the micellar solu-tion discussed earlier. The difference between the two isthat microemulsions can form below CMC and, as aresult, solubilization can commence at surfac'tant con-centrations below such CMC,' thus reduci~g precipita-tion and maybe even eliminating it.

It is clear from the discussed results that surfactants ofthe sulfonate type can undergo precipitation andredissolution on interacting with solutions containingdissolved inorganic ions and that such phenomena c~playa major role in producing a maximum in the abstrac-tion isotherms.

monovalent ions was capable of dissolving bisulfonateprecipitates. Addition of NaCI thus caused the redissolu-tion of the calcium sulfonate precipitates. Addition of Naions is believed to lower the CMC of the sulfonate andgive rise to the formation of micelles and in turn to thedissolution of the precipitate in them.

S. Possible mechanisms suggested for the redissolu-tion phenomenon are: solubilization of precipitates bymicelles; redissolution by coh1plexation; and redisper-sion of the coagulated colloids caused by the develop-ment of charge resulting from the adsorption ofsulfonate. Solubilization by micelles is considered to bethe predominant mechanism for the systems studiedhere.

6. Precipitation of surfactant because of its interactionwith dissolved mineral species is confirmed to contributeto the presence of maximum observed in the "adsorp-tion" isotherms. Because of this, it is recommended thatprecipitation and abstraction experiments be conductedunder identical environments to calculate the .actual ad-sorption densities on minerals.

7. Addition of oil was found to reduce (in some casescompletely eliminate) surfactant precipitation, possiblybecause of the partitioning of the surfactant between theoil and the aqueous phases.

AcknowledgmentsWe gratefully acknowledge the suppon of the U.S. DOE(DE-AC-19-79BC-l0082), the Natl. Science Foundation(ENG- 78-11776), Amoco Production Co., Chevron Oil

SOCIETY OF PETROLEUM ENGINEERS JOURNAL

Conclusions

1. The study of kinetics of precipitation as a functionof electrolyte concentration showed the time for com-plete precipitation to depend on the sulfonate concentra-tion, with precipitation continuing to take place for hourswhen the concentration of inorganic electrolyte is low.

2. Monovalent cations such as K + caused theprecipitation of sulfonates on increasing the sulfonateconcentration. Multivalent ions such as Ca + + andAl + + + also precipitated the sulfonate, but in these cases

the precipitate redissolved on further increasing thesulfonate concentration. The concentration at whichcomplete redissolution occurs appears to be a function ofthe valency of the multivalent ion.

3. The solubility products of the salts of sulfonate weredetermined from the turbidity measurements; calciumand aluminum dodecylbenzenesulfonates have thesolubility products of 2x10-11 and 4x10-19respectively.

4. In the studies with solutions containing mixwres ofmono- and multivalent ions. addition of ev.en

238

~

2:t.

Z0

~Go~0In0C

Z0

~Ct-

~U1&1~Go

Z0

~uc~t-InmC

15

10

,

Field Research Co., Exxon Research and EngineeringCo., Gulf R&D Co., Marathon Oil Co., Mobil R&DCo., Shell Development Co., SOHIO, Texaco Inc., andUnion Oil Co. of California. We also thank K.P. Anan-thapadmanabhan for helpful discussions.

9. Home, R. W. and Ottewil, R.H.: "Studies on the Plttipitation ofMetallic Salts of Tetradecyl Sulfate," Proc., World Cong~ss onSurface Active Agents (1960) 1-2, 103-108.

10. O'Brien, E.F. and Wiers, G.H.: "Plttipitation Boundaries inCalcium-Alkylbenzenesulfonate Systems," hoc., 48th Nat!. Col-loid Symposium, Austin, TX (1974) 220.

II. "Adsorption from Flooding Solutions in Porous Media," annual~11 submitted to the Nat!. Science Foundation and a consoItiumof supponing industrial organizations, Columbia U., New Yolt.City (July 1978).

12. McBain, J. W.: Advanc~s in Colloid Sci~nc~, Wiley Interscience,New Yolt. City (1942) I.

13. Kitchener, J.A.: "Mechanisms of Adsorption from Aqueous Solu-tions: Some Basic Problems," J. Photographic ScierII:e (I96S) 13,IS2-S8.

14. Trogus, F.J., Schechter, R.S., and Wade, W.H.: "Adsorption ofMixed Surfactant Systems," J. P~t. Tech. (June 1979) 769-78.

SI Metric Conversion Factondyne x 1.0* E-02 = InN

ft x 3.048* E-OI = mOF (OF-32fl.8) = °Cgal x 3.785412 E-03 = m3in. x 2.54* E+OO = cmL x 1.0* E+OO = drn3

Ibm x 4.535 924 E-OI = kgmil x 2.54* E-02 = om

'~~.8XacI. SPEJOr;g;n8I man~ ~ ~ 8oci8Iy of ~m Engi.-s oIIk» Oct. 31, 1178.Paper 8CC8IM8d fat pubIC8ticwI M8fct1I, 1M3 ~ ~ '-'- Oct 11,1M3 PIPer (SPE 8283) fWst ~ .. ItI8 1971 SPE ~ T8ctw*8I cor..-.nce 8Id ExIIiIiIm IWkI in L8 V8g8 s.pc. 23-26

ReferencesI. Hanna. H.S. and Somasundaran, P.: "Physico-Chemical Aspects

of Adsorption at Solid/Liquid Interface, Pan I," Impro~d OilReco~ry by Surfactanl and Polymer Flot>ding, Academic ~ssInc., New York City (1911) 205-51.

2. Hanna, H.S. and Somasunda~n, P.: "Mahogany Sulfonate/Be~Sandstone, Pan II," Impro~d Oil Reco~f)' by Slufactont andPolymer Flooding. Academic ~ss Inc., New York City (1971)233-13.

3. Somasunda~n, P. and Hanna, H.S.: "AdSOrption of Sulfonateson Reservoir Rocks," Soc. Pel. Eng. J. (Aug. 1919) 221-32.

4. TNshenski, S.P., Dauben, D.C., and PaJrish, O.K.: "MicellarFIocxting-Fluid Propagation, Interaction, and Mobility," Soc.Pel. Eng. J. (Dec. 1914) 633-42; Trans.. AlME, 257,

5 . Bae, J. H. and Petrick, C. B.: " Adsorption/Retention of Petroleum

Sulfonates in Be~ Cores," Soc. Pet. Eng. J. (Oct. 1971)353-51.

6. Powers, G.W.: The VollUnetric Detennination of Organics.Ilfonates or Sulfanates by the Double Indicator Method, AmocoMethod C-225 (1910).

1. Reid, V.W., Longman, G.F., and Heinerth, E.: "Determinationof Anionic-Active Detergents by Two-Phase Titration," Tenside(1961) 4, No.9, 292-94.

8. Reid, V.W., Longman, G.F., and Heinenh, E.: "Determinationof Anionic-Active Detergents by Two-Phase Tit~tion," Tenside(1968) 5, No. 3-4, ~96.

239APRIL 1984